1. Field of the Invention
The present invention relates to an image processing apparatus for converting input color image signals to image recording signals that are sent to an image forming apparatus such as a color printer, the input color image signals including image data that is obtained through scanning of a photographic film or acquired from a digital still camera.
2. Description of the Related Art
Recent years have seen advances in computer technology, improvements in communication networks and the introduction of mass storage media, accompanied by widespread use of scanners and digital still cameras. The trend has given impetus to a growing need for processing photographic image data, i.e., the need for photo image data to be printed out with high quality.
In the area of photographic films, images are output less often in analog fashion today. Instead, image processing and editing are carried out increasingly on digitized images to comply with a need for enhanced-quality, multi-functional image print-out.
The photographic image data to be printed out typically comprises so-called memory colors of characteristic image portions such as the color of the human skin, the blue of the skies and the green of the greenery. A need has been recognized to reproduce such characteristic portions in visually preferred or desired colors.
The need for such memory color reproduction is addressed illustratively by Japanese Published Unexamined Patent Application No. Hei 6-121159. The publication discloses a method for extracting memory colors from an image and converting the extracted portions into visually desirable colors that were previously determined by sensor evaluation. The extracted portions with their colors thus determined are then printed out.
Image forming apparatuses such as printers and image display units have different ranges of color reproduction from one device to another. In particular, printers differ significantly from display units in terms of color reproduction ranges. This can lead to cases where input image data acquired from an image input source such as a scanner is not necessarily printed exactly as displayed, part of the input image data being left out upon print-out.
FIG. 17 is a graphic representation showing differences in color reproduction ranges between devices. In FIG. 17, a region op indicates a typical color reproduction range of a color printer (Kodak XL7700), and a region Gm stands for a representative color reproduction range of an ordinary ROB color video monitor. The figure sketches the color reproduction ranges on an a*b* plane at L*=50.0 in a CIE.multidot.L*a*b* color space.
In FIG. 17, the image data at point Pi cannot be printed out as such because it is outside the color reproduction range Gp of the printer. To have the image data at point Pi output by the printer requires additional procedures of operation or data conversion.
Several such procedures have been proposed so far. For example, Japanese Published Unexamined Patent Application No. Hei 7-298073 discloses ways to "clip" image data outside a given color reproduction range in a chroma direction such that the data will be located on a boundary of the target range. More specifically, the image data at point Pi in FIG. 17 is converted to image data at point Po1 where a line 1 connecting point Pi with the origin of the a*b* coordinates intersects the boundary of the color reproduction range Gp, the data conversion being performed in such a manner that the lightness and hue of the data at point Po1 match those of the data at point Pi.
Japanese Published Unexamined Patent Application No. Hei 5-115000 discloses another technique. The disclosure involves establishing a region in which to reproduce desired data inside the color reproduction range of the printer without chroma contraction; image data outside the range is partially contracted in chroma.
Japanese Published Unexamined Patent Application Nos. Hei 6-253138 and Hei 6-253139 disclose a method whereby specific points in color space are explicitly mapped, the remaining portions being mapped by interpolation or like manner. According to this method, the image data at point Pi in FIG. 17 is converted to image data at point Po2 on a boundary of the color reproduction range Gp.
Japanese Published Unexamined Patent Application No. Hei 6-189121 discloses another method. The disclosed method utilizes evaluation functions supplemented with deviations weighted in terms of lightness, chroma and hue between monitor-displayed coloration and print colors in response to input signals. These functions are used to optimize color converting parameters, whereby the target color reproduction range is contracted for preferred sensory perception.
The method disclosed by the above-cited Japanese Published Unexamined Patent Application No. Hei 6-121159 appears to be effective in reproducing memory colors in visually preferable coloration upon print-out of photographic image data.
However, the disclosed method has a disadvantage stemming from a color correcting process performed on the extracted memory color regions, the process being different from the one on the remaining regions. The dissimilar process can produce a discontinuity between the memory colors and the remaining colors as shown in FIG. 18A, generating a false contour or other flaws in color reproduction.
Japanese Published Unexamined Patent Application No. Hei 7-298073 cited above resorts to the clipping of image data in addressing a narrower color reproduction range of the printer than that of the display unit. However, the method of clipping image data outside the color reproduction range in the direction of chroma to relocate the data onto a boundary of the range can lead to a chroma fusion in highly saturated portions, as indicated by a linear segment 3 in FIG. 18B. The result can be a significantly degraded quality of output images.
The technique disclosed in the above-cited Japanese Published Unexamined Patent Application No. Hei 5-115000 aims to establish a region in which to reproduce image data inside the color reproduction range of the printer without chroma contraction, while image data outside the range is partially contracted in chroma. This method, while not as conspicuously as the above-mentioned clipping method, can still cause a chroma fusion in highly saturated portions as shown by a linear segment 4 in FIG. 18B, resulting in a degraded quality of output images.
The method disclosed in Japanese Published Unexamined Patent Application Nos. Hei 6-253138 and Hei 6-253139 eases any chroma fusion in highly saturated portions. Still, this method has a disadvantage, as evident from a curved segment 9 in FIG. 18C, of causing a warped hue and of disturbing linearity of the chroma gradation characteristic. In addition, the method has difficulty in guaranteeing the continuity of colors in cases where there are many points to be matched in color.
The technique of the above-cited Japanese Published Unexamined Patent Application No. Hei 6-189121 has a disadvantage similar to that of the clipping method regarding output chroma characteristics as opposed to input chroma. That is, the technique also tends to cause a chroma fusion in highly saturated portions, although not as severely as the clipping method, as indicated by a broken line curve 5 in FIG. 18B. The disclosed technique has another disadvantage of requiring a great deal of effort in determining the necessary evaluation functions. Specifically, color converting parameters need to be determined by establishing a large number of points as evaluation points in color space. For each evaluation point, a weighting factor needs to be determined through sensor evaluation.
As the digital photo system is coming into general use, there will be a growing number of cases in which users bring their photo films or CD-ROMs into local print shops and later receive prints there derived from the submitted materials. In such cases, there will conceivably be more users wishing to get better color reproduction in their prints than those who prefer having their prints coincide with monitor-displayed colors.
Aged deterioration of photo films or their inadequate exposure, as well as diverse image input devices to be dealt with, can contribute to color balance variations from one print image to another. It follows that faithfully printing out input data according to its color balance can lead to undesirable color reproduction of prints if the color balance of the input data has been subsequently changed.
As outlined above, there have yet to be developed color image processing techniques for converting digital photo data into colors exactly as desired by users. The method for contracting the color reproduction range, extensively used so far, rests on the precondition that print colors coincide visually with monitor-displayed colors. The requirement involves having the color reproduction range contracted in the chroma direction while keeping the level of overall saturation as high as possible. This means theoretically that highly saturated portions tend to produce a fusion of chroma gradation.
It may happen that a color imbalance on the monitor results in flawed color reproduction thereby. This will make it impossible to acquire the user's preferred colors even if the print colors match the monitor-displayed colors.
The hitherto-proposed technique of extracting memory colors of the humans and subjecting the extracted colors to a different color correcting process apparently improves reproduction of the memory colors. But the technique is deficient in overcoming the discontinuity between the memory colors and other colors or in eliminating the generation of a false contour.
The point is not that print colors should match some extraneous criteria but that the colors are to be reproduced as preferred by users. This simply requires making colors attracting users' attention such as memory colors coincide with what the users keep in their memory.
It happens frequently that the need for reproducing memory colors and the need for preventing chroma fusion in highly saturated portions are mutually exclusive. A trade-off needs to be made between these two requirements when color converting characteristics are determined.
There is room for variations in the colors that are kept in the human memory. This means that, unlike under conventional schemes, memory colors need not necessarily match predetermined values. What is more important is for hue and chroma settings to be brought to better levels than before.
In order to implement gradation without chroma fusion in highly saturated portions, it is imperative to utilize the widest possible range of color reproduction on the part of the image forming apparatus. To make memory colors coincide with what users keep in their memory requires that a memory color range be extracted from input image data with the characteristics of the input data taken into consideration.